The present in vent ion relates to a preheating circuit for detecting the filament temperature of fluorescent lamps, and more particularly to a circuit indirectly detecting a filament temperature to ensure that filaments operate at a thermionic emission temperature.
Properly preheating filaments becomes considerably necessary to avoid deteriorating the lamp life. Igniting a lamp at a low filament temperature requires a relatively high ignition voltage, causing bombardment and resulting in extremely sputtering on filaments. On the other hand, overheating the filaments will cause their coating material over evaporating and thermal shock. Both of the two improper preheating conditions engender sputtering and shorting the life of the lamp. Lamp filaments must reach their emission temperature at starting stage to minimize electrode sputtering. The preheating ratio (xcex3=Rh /Rc) of the hot resistance (Rh) of the electrodes to their cold resistance (Rc) is an index in knowing a n approximate emission temperature, and the electrodes with such a ratio means that it reaches a temperature high enough for thermionic emission.
FIGS. 1(a)xcx9c(c) show three typical preheating circuits for fluorescent lamps. Please refer to FIG. 1(a). The preheating circuit is implemented by using the characteristic that the resistance of the positive temperature coefficient (PTC) of the resistor R1 is increased with increasing temperature to preheat the filaments. When the resistance of the resistor R1 is low at a low temperature, most of the preheating current flows through the capacitor C1 and the resistor R1. At this time, the circuit operates at a preheating frequency to preheat the filaments. When the resistance of the resistor R1 increases with the increasing temperature, more current flows from the capacitor C1 to the capacitor C2. The disadvantage of the preheating circuit is that the filaments are hard to operate at a thermionic emission temperature because the preheating time depends on the variation of the positive temperature coefficient resistance.
Referring to FIG. 1(b), the resistors R3 and R4 in series form a voltage divider. The voltage V1 in the voltage divider turns on the switching element Q2 and the switching element Q2 is in parallel with the capacitor C4. Therefore, the voltage across the lamp is low. When the current flows through the resistor R2 to charge the capacitor C3 until the capacitor voltage of the capacitor C3 reaches the breakdown voltage of the diode D1, the switching element Q1 is turned on and the switching element Q2 is forced to be turned off. The capacitance of the capacitor C3 is adjusted to determine the charging time of the capacitor C3 to control the preheating time so as to let the filament temperature is high enough. Therefore the preheating time is determined by the amount of the charges on the capacitor C3. If the initial voltage of the capacitor C3 is high, the charging time for reaching the breakdown voltage of the diode D1 is shorter. On the other hand, the initial voltage of the capacitor C3 is zero, the charging time for reaching the breakdown voltage of the diode D1 is the longest. Therefore, the phenomenon of overheating the filaments or igniting a lamp at a low filament temperature also exists because the preheating time depends on the amount of the charges on the capacitor C3 but does not depend on the filament temperature.
As shown in FIG. 1(c), the charging time of the RC circuit is used to control the preheating time. When the voltage of the capacitor C5 is not charged to the breakdown voltage of the diode D2, the circuit operates in higher frequency and the lamp voltage is not high enough to ignite the lamp. And the resonant current is used to preheat the filament. The drawback is same as described in FIG. 1(b). The phenomenon of overheating the filaments or igniting a lamp at a low filament temperature also exists because the preheating time depends on the amount of the charges on the capacitor C5 but does not depend on the filament temperature.
Otherwise, U.S. Pat. No. 5,920,155 discloses an electronic ballast for discharge lamps which sets a filament current and a voltage across a discharge lamp at their suitable operational levels according to respective operational states of the discharge lamps, and which also provides a sufficient dimming function even when the lamp is of a slim type. However, it is not mentioned how to dynamically adjust the preheating time. Therefore, the filaments are not sure to operate at a thermionic emission temperature. Thus, the preheating circuit needs to be improved to overcome the above problem.
It is therefore an object of the present invention to propose a preheating circuit for a fluorescent lamp. The preheating circuit includes a filament detecting circuit for indirectly detecting a filament resistance in a fluorescent lamp by measuring a filament voltage and a filament current, a pulse generation circuit for providing pulses of one of a first frequency and a second frequency determined by the detected filament resistance and a specific filament resistance, and a filament resonance circuit operating the fluorescent lamp at an operating frequency determined by the pulse generation circuit so that the filament resonance circuit operates at the first frequency to preheat the fluorescent lamp when the detected filament resistance is smaller than the specific resistance and the filament resonance circuit operates at the second frequency to operate the fluorescent lamp when the detected filament resistance is one of a first value being larger than and a second value being equal to that of the specific resistance.
According to an aspect of the present invention, the first frequency is a preheating frequency xcfx89s(ph).
Preferably, the second frequency is a switching frequency xcfx89s(fl) at full load.
Preferably, the specific resistance is a hot filament resistance Rh which is an index to preheat the fluorescent lamp when the detected filament resistance Rf is smaller than the hot filament resistance Rh.
Preferably, the hot filament resistance Rh is xcex3 times a cold filament resistance RC where xcex3 is a preheating ratio and xcex3 greater than 1.
Preferably, the filament detecting circuit includes a first series circuit of a secondary winding of a transformer and a first diode electrically connected in parallel to a first smoothing capacitor and a first resistor for generating a first DC output voltage, a second series circuit of a filament resistor and a second diode connected in parallel to a second smoothing capacitor and a second resistor for generating a second DC output voltage, and a comparator having an inverting input electrically connected in parallel to the first smoothing capacitor, and a noninverting input electrically connected in parallel to the second smoothing capacitor for providing a switching signal to the pulse generation circuit for generating the operating frequency.
Preferably, the first DC output voltage is in proportion to a secondary voltage Vxe2x80x2Lr of the secondary winding of the transformer and the second DC output voltage is in proportion to a filament voltage VRf across the filament resistor.
Preferably, the secondary voltage Vxe2x80x2Lr equals to xcex3Rc*VLr/xcfx89s(ph)Lr where VLr is a primary voltage of the primary winding of the transformer, and Lr is an inductance of the primary winding of the transformer.
Preferably, the filament voltage VRf equals to Rf*VLr/xcfx89s(Ph)Lr.
Preferably, the filament resonance circuit operates at the preheating frequency xcfx89s(ph) to preheat the fluorescent lamp when the detected filament resistance Rf is smaller than the hot filament resistance Rh while the filament resonance circuit operates at the switching frequency xcfx89s(fl) to operate the fluorescent lamp when the detected filament resistance Rf is one of a first value being larger than and a second value being equal to that of the hot filament resistance Rh.
Preferably, the filament resonance circuit operates at the preheating frequency xcfx89s(ph) to preheat the fluorescent lamp when the filament voltage VRf is smaller than the secondary voltage Vxe2x80x2Lr while the filament resonance circuit operates at the switching frequency xcfx89s(fl) to operate the fluorescent lamp when the filament voltage VRf is one of a first value being larger than and a second value being equal to that of the secondary voltage Vxe2x80x2Lr.
The present invention may best be understood through the following description with reference to the accompanying drawings, in which: